1. Field of the Invention
The present invention relates to a solid-state imaging device, in particular, a solid-state imaging device in which a driving pulse is applied to a vertical transfer electrode via a shunt wiring, and a method for manufacturing the same. More specifically, the present invention relates to a shape of a light-shielding film and a method for connecting the shunt wiring and the vertical transfer electrode.
2. Description of Related Art
In recent years, demands for solid-state imaging devices have been increasing as imaging devices for use in digital still cameras and digital video cameras. Further, due to a request that portable terminal devices such as a cellphone additionally have a camera function, the demands for solid-state imaging devices have been expanded. Along with this, there also is a growing demand for high-quality images. In order to enhance the image quality of a solid-state imaging device, it is required both to increase the number of pixels and to improve the sensitivity by increasing the S/N ratio.
A prerequisite for increasing the number of pixels of a solid-state imaging device is to enhance the operating speed of the solid-state imaging device. In order to enhance the operating speed of a CCD (Charge Coupled Device) solid-stage imaging device, it is required to transfer signal charges from an imaging portion to a charge accumulating portion at a high speed.
To achieve a high charge transfer rate, it has been proposed to connect corresponding vertical transfer electrodes with a common shunt wiring that also serves as a light-shielding film, thereby reducing the influence of an electrical resistance of the vertical transfer electrodes. Here, in many cases, an aluminum film, which is a preferable material for use in the shunt wiring that also serves as a light-shielding film, is formed on a flattening film since it is difficult for it to be formed in a step portion. When a light-shielding film is formed on a flattening film, it is necessary to prevent smear that occurs due to oblique incident light that cannot be blocked. To this end, in addition to the shunt wiring made of an aluminum film that also serves as a light-shielding film, another light-shielding film made of a high melting point metal such as tungsten is formed so as to cover a side wall portion of the vertical transfer electrode, as disclosed in JP H5 (1993)-243537 A, for example.
FIG. 12 is an enlarged view showing a planar configuration of an imaging portion of such a conventional solid-state imaging device. FIGS. 13A-B are enlarged cross-sectional views of portions where a shunt wiring that also serves as a light-shielding film is formed in the conventional solid-state imaging device. FIG. 13A shows a cross-sectional structure taken along a line a-a in FIG. 12, and FIG. 13B shows a cross-sectional structure taken along a line b-b in FIG. 12.
As shown in FIGS. 12 and 13A-B, the imaging portion of the conventional solid-state imaging device includes a plurality of photoelectric conversion portions 101 formed in a matrix on a semiconductor substrate 122 and a plurality of vertical transfer channels 102 formed so as to extend in a column direction, i.e., a vertical direction in FIG. 12, between the photoelectric conversion portions 101 adjacent in a row direction, i.e., a horizontal direction in FIG. 12.
Further, a pair of vertical transfer electrodes 111A and 111B extending in the row direction are formed so as to sandwich each of the photoelectric conversion portions 101 in the column direction. The vertical transfer electrodes 111A and 111B respectively have downward and upward protruding portions in FIG. 12 in portions between vertical columns of the photoelectric conversion portions, i.e., regions where the vertical transfer channels 102 are formed. The protruding portions have their ends overlapping each other, with an insulating film not shown interposed therebetween.
One light-shielding film 113A is formed so as to cover vicinities of end portions and side wall portions of the protruding portions of the vertical transfer electrodes 111A and 111B on the left side in FIG. 12, and another light-shielding film 113B is formed so as to cover vicinities of end portions and side wall portions of the protruding portions of the vertical transfer electrodes 111A and 111B on the right side in FIG. 12. A first insulating film 115 is formed between the vertical transfer electrode 111 and the light-shielding film 113.
On the light-shielding film 113, a shunt wiring 114 that also serves as a light-shielding film is formed via a second insulating film 116 so as to cover the vertical transfer channel 102. The shunt wiring 114 that also serves as a light-shielding film is connected to the corresponding vertical transfer electrode 111 with a contact portion 121. The contact portion 121 is made of the same metal material as that for the shunt wiring 114 that also serves as a light-shielding film, and is formed by a sputtering method simultaneously with the shunt wiring 114.
In the example shown in FIG. 12, the vertical transfer electrode 111B arranged below the central upper photoelectric conversion portion 101 in the figure is connected to the shunt wiring 114 that also serves as a light-shielding film on the right side in the figure, and the vertical transfer electrode 111A arranged above the central lower photoelectric conversion portion 101 in the figure is connected to the shunt wiring 114 that also serves as a light-shielding film on the left side in the figure. A driving pulse for transferring a charge obtained by photoelectric conversion is applied directly to the vertical transfer electrode 111.
In the conventional solid-state imaging device as described above, since the shunt wiring 114 that also serves as a light-shielding film is connected electrically to the corresponding vertical transfer electrode 111 with the contact portion 121, the shunt wiring functions as a pulse transmission line, contributing to a lower resistance of the vertical transfer electrode. Accordingly, a delay in propagating signal charges can be suppressed, especially in a central portion of the solid-state imaging device, as compared with the case where a driving pulse for photoelectric conversion and charge transport is applied using only the vertical transfer electrode 111. As a result, it is possible to realize a solid-state imaging device that can be operated at a higher speed.
However, although the above-described conventional solid-state imaging device can be operated at a high speed, it has a problem with a withstand voltage between the light-shielding film 113 and the contact portion 121 formed simultaneously with the shunt wiring 114 that also serves as a light-shielding film, as pixels become finer.
More specifically, the electrical withstand voltage between the light-shielding film 113 and the contact portion 121 is defined by a distance between these two metal members, and a portion with the smallest distance cis a problem. For example, as shown in FIG. 13B, it is assumed that t1 and t2 represent the distances on the left side and the right side, respectively, in the figure. In the case of FIG. 13B where t1<t2, the distance between the light-shielding film 113A positioned on the left side of the vertical transfer electrode 111 and the contact portion 121 is a problem.
Since the contact portion 121 is manufactured simultaneously with the shunt wiring 114 that also serves as a light-shielding film by a sputtering method or the like as described above, there is a certain limit to the accuracy of its forming position due to variations during a manufacturing process. However, to ensure such design dimensions that can absorb variations during a manufacturing process, the light-receiving portion has a smaller area, and the effective sensitivity is reduced, resulting in a lower S/N ratio. On the other hand, to ensure that the light-receiving portion has a sufficient area, pixels have to be made larger. Accordingly, it is impossible to achieve finer pixels, contrary to the request for a solid-state imaging element to have an increased number of pixels.
Further, according to the above-described conventional solid-state imaging device, although the vertical transfer electrodes are joined with the shunt wiring, thereby decreasing their electrical resistance, a signal for charge transfer is applied from the vertical transfer electrodes. Therefore, a path that allows the charge transfer signal to pass through the vertical transfer electrodes cannot be made shorter, and thus a sufficient high-speed operation cannot be achieved.